(1) Field of the Invention
The present invention relates to a fixing device having guide members that suppress a curl and a wave in a recording sheet, and to an image forming apparatus comprising the fixing device.
(2) Description of the Related Art
In recent years, the mainstream structure of image forming apparatuses that incorporate electrophotography is such that each apparatus has a compact body with a paper feed mouth, a transfer position and a fixing position arranged substantially on a straight line extending in the direction of the height of the apparatus, so as to minimize the length of the path along which paper feeding, transferring, fixing and paper discharge are performed.
However, image forming apparatuses with such a conveyance path extending in the direction of the height of the apparatuses (hereinafter also referred to as “image forming apparatuses for height-direction conveyance”) have a problem that a recording sheet is easily curled or waved after passing through the fixing device (a wave in the recording sheet is a phenomenon in which there is a repetition of small curls in the recording sheet).
In general, immediately after the recording sheet passes through a fixing nip formed in the fixing device, the melted toner is not completely dried out, and moisture is evaporated from the recording sheet as a result of fixing by heat and pressure. This renders the recording sheet feeble and very soft. If the temperature of the recording sheet decreases thereafter, the shape of the recording sheet at that moment tends to remain.
Assume a case where the recording sheet is conveyed in a horizontal direction to pass through the fixing device and then is discharged in the horizontal direction. In this case, the recording sheet is in a stable posture after passing through the nip due to gravity; hence, the recording sheet is not easily curled or waved. On the other hand, in the above-described image forming apparatuses for height-direction conveyance, since the recording sheet is conveyed upward after passing through the fixing nip, the recording sheet is in an extremely unstable posture, and hence the curl/wave phenomena easily occur.
A curled or waved recording sheet has a very undesirable appearance. In addition, when a large number of recording sheets are discharged onto a discharge tray in a stack, curls or waves in the recording sheets make it difficult to line up the recording sheets. In the worst case, a curl or a wave in a recording sheet could cause a jam.
To address the above problems, fixing devices in conventional image forming apparatuses are structured as follows. As shown by a fixing device 500 of FIG. 7, a pair of intermediate rollers 504 is placed in a position that is between (i) a fixing nip formed by a heating roller 501 and a pressurizing roller 502 and (ii) a pair of discharge rollers 506, and that is close to the fixing nip. Immediately after the recording sheet passes through the fixing nip, the recording sheet is corrected in posture by the pair of intermediate rollers 504 applying suitable tension to the recording sheet. Thereafter, the recording sheet is conveyed to the pair of discharge rollers 506 along a conveyance path 505.
However, with the structure of the above-described conventional fixing devices, existence of the pair of intermediate rollers 504 is inevitable. Accordingly, a large number of components are required in the conventional fixing devices, and a cost increase is unavoidable. Such conventional fixing devices are against the idea of making image forming apparatuses compact.